Congenital heart defects are the most common birth defects in humans, affecting approximately 1% of all live births. The mammalian heart first forms as a linear tube from cells of the First Heart Field. Subsequently, the heart tube expands rapidly due to the addition of cells from a population of later differentiating cardiac progenitors, referrd to as the Second Heart Field (SHF). Cells from the anterior domain of the SHF, referred to as the anterior heart field (AHF), are added to the arterial pole and form the majority of the right ventricle and outflow tract. MEF2C is a critical regulator of the AHF transcriptional network, and mice lacking Mef2c have profound defects in the formation of the right ventricle and outflow tract. However, the transcriptional targets of MEF2C involved in AHF development are largely unknown. Preliminary studies establish that inactivation of Mef2c in mice results in death at birth due to ventricular septal defects and outflow tract alignment abnormalities. To gain insight into the molecular targets downstream of MEF2C responsible for these profound congenital heart defects, RNA-sequencing was performed on outflow tracts from mouse embryos collected at embryonic day 10.5. These studies identified two previously unknown MEF2C target genes as completely dependent on MEF2C for expression in the developing outflow tract. The Myosin-binding protein c1 (Mybpc1) and Teratoma-derived growth factor 1 (Tdgf1) genes were each strongly expressed in control outflow tracts at mouse embryonic day 10.5, but the expression of both genes was abolished in the absence of MEF2C. Tdgf1 encodes the Nodal co-receptor Cripto, a molecule involved in establishing symmetry in the developing embryo. Moreover, the TDGF1 gene is associated with ventricular septal defects and other congenital heart defects in humans. Mybpc1 encodes a member of the Myosin-binding protein C family of sarcomeric proteins that are important in forming actomyosin cross bridges and sarcomere integrity. MYBPC1 mutations in humans are associated with skeletal muscle defects, including lethal congenital contracture syndrome and congenital arthrogryposis. Mybpc1 may also be associated with congenital heart defects, but no animal models have been developed, making mechanistic in vivo studies of this gene and its function in cardiac and skeletal muscle development challenging. This proposal will test the hypotheses that Tdgf1 is a direct transcriptional target of MEF2C via a novel cardiac-specific enhancer and that Tdgf1 function is required in the AHF for proper outflow tract alignment using mouse transgenic and genetic approaches and cell culture-based experiments. This proposal will also test the hypothesis that Mybpc1 is essential for sarcomere integrity in both cardiac and skeletal muscle using genetic approaches in mice combined with extensive histological analyses.